1. Field of the Invention
The invention relates to an exhaust gas purification apparatus for an internal combustion engine.
2. Description of the Related Art
An internal combustion engine is known in which a NOx adsorption-reduction catalyst that adsorbs NOx contained in the incoming exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases the adsorbed NOx when the air-fuel ratio of the exhaust gas is equal to the stoichiometric air-fuel ratio or rich is provided in an exhaust gas passage. The NOx adsorption-reduction catalyst includes a noble metal catalyst made of platinum Pt and a NOx absorbent. When the air-fuel ratio of the exhaust gas is lean, the NOx contained in the exhaust gas, that is, the NO contained in the exhaust gas is oxidized into NO2 on the platinum Pt and then absorbed in the NOx absorbent in the form of nitrate ions NO3−.
On the other hand, when releasing the absorbed NOx from the NOx absorbent and reducing the released NOx, the air-fuel ratio of the exhaust gas entering the NOx adsorption-reduction catalyst is made rich. As the air-fuel ratio of the exhaust gas is made rich, the oxygen concentration of the exhaust gas decreases, so that the NOx absorbed in the NOx absorbent in the form of nitrate ions NO3− becomes NO2 and moves to the surface of the platinum Pt, and the NO2 is then reduced by the unburned HC and CO contained in the exhaust gas.
The air-fuel ratio of exhaust gas can be made rich by supplying additional fuel into the respective combustion chambers or by adding fuel into the exhaust gas passage. In either case, if fuel is added such that the added fuel enters the NOx adsorption-reduction catalyst in the form of a fuel gas, NOx is immediately released from the NOx adsorption-reduction catalyst and then reduced in response to the air-fuel ratio of the exhaust gas being made rich. However, this does not happen if fuel is added into the exhaust gas passage in the form of fine fuel droplets and then adheres to the NOx adsorption-reduction catalyst in the form of fuel duplets.
That is, if the fuel that has been added to make the air-fuel ratio of the exhaust gas rich adheres to the NOx adsorption-reduction catalyst in the form of fuel droplets, the platinum Pt supported on the NOx adsorption-reduction catalyst is covered by the liquid fuel. When the platinum Pt is covered by the liquid fuel, the oxygen contained in the exhaust gas cannot reach the surface of the platinum Pt. Therefore, the liquid fuel on the platinum Pt cannot be oxidized properly. If the liquid fuel cannot be oxidized properly, the oxygen in the exhaust gas is not sufficiently consumed. Thus, the oxygen concentration does not decrease sufficiently, and therefore NOx is not released from the NOx absorbent sufficiently. Further, in this case, because the liquid fuel is not effectively vaporized, the amount of unburned HC in the exhaust gas becomes insufficient, and therefore the released NOx can not be reduced sufficiently.
In view of this, the present inventors, during their study, have focused on the oxygen adsorption capability of palladium Pd and discovered the following. That is, if palladium Pd is supported on the NOx adsorption-reduction catalyst as noble metal as well as platinum Pt, the oxidization of the liquid fuel on the NOx adsorption-reduction catalyst is promoted by a large amount of oxygen adsorbed in the palladium Pd, and the heat generated by this oxidization accelerates the vaporization of the liquid fuel on the platinum Pt, thus promoting the release of NOx from the NOx absorbent.
If the amount of palladium Pd is increased and the amount of platinum Pt is reduced, the vaporization of the liquid fuel on the platinum Pt is promoted by the reaction heat generated through the oxidization of the palladium Pd. In this case, however, because the amount of platinum Pt is small, the NOx release effect is weak, and therefore NOx cannot be released effectively. On the other hand, if the amount of palladium Pd is reduced and the amount of platinum Pt is increased, the oxidization of the liquid fuel on the platinum Pt is not promoted by the reaction heat generated through the oxidization of the oxygen adsorbed in the palladium Pd, and therefore the NOx release effect becomes weak despite the increased platinum Pt. Thus, in this case, too, NOx cannot be released effectively.
As such, it is clear that the effective release of NOx can be achieved only when the ratio between the amount of platinum Pt and the amount of palladium Pd is in a given proper range that is not excessively high nor excessively low. With regard to this point, Japanese Patent Application Publication No. 2003-205245 (JP-A-2003-205245) describes a particulate filter on which platinum Pt and palladium Pd are supported such that 1 gram of platinum and 1 gram of palladium Pd are supported per liter of the volume of the filter body. In this case, the ratio of the mole number of the platinum Pt to the sum of the mole numbers of the platinum Pt and the palladium Pd is approx. 35.7. However, with such a mole ratio, the amount of the palladium Pd is too large as compared to the amount of the platinum Pt. Therefore, NOx cannot be released effectively.
Meanwhile, the present inventors discovered the following as a result of their continuous study on the effect of palladium Pd. That is, if the ratio between the amount of platinum Pt and the amount of palladium Pd is set appropriately, the effective release of NOx can be accomplished. However, when the temperature of the NOx adsorption-reduction catalyst is low, the palladium Pd impedes the absorption of NO2 in the exhaust gas into the NOx absorbent. Thus, when the temperature of the NOx adsorption-reduction catalyst is low, such as immediately after engine start, the NOx removal rate decreases.